In Wavelength Division Multiplexed (WDM) optical links it is difficult to assure that signals arriving at each channel's photodetector have a power level that is within the receiver's dynamic range. Even for simple point-to-point links, flattening filters are used in the Erbium Doped Fiber Amplifiers (EDFA's), MUX/DEMUX components' profiles of attenuation vs. wavelength must be trimmed, and the system must be carefully monitored to ensure that large inter-channel differences in concatenated connector and splice losses are not accumulated.
Typically, all WDM channels are amplified in a single amplifier, with the single amplifier being optimized for gain flatness. However, there are different power levels in each channel due to differences in accumulated channel losses at different frequencies. Variable Optical Attenuators (VOA's) are used in the respective channels to compensate for the losses. The VOA's require frequent adjustment to maintain required power levels, and if the power level in a given channel drops below a minimum level, a transponder is required in the line to increase the power level to the required level.
Thus, there is a need to be able to automatically readjust the power level on a per-channel basis so that the photodetector at the optical receiver receives a signal with an adequate Optical Signal to Noise Ratio (OSNR) and amplitude to achieve a desired Bit Error Rate (BER), but not so high a power level that the optical receiver or the electronics to follow are saturated.